This invention relates to a method of producing a cerium-activated yttrium orthoaluminate (YAlO.sub.3) phosphor which emits ultraviolet radiation and has a short decay time.
Cerium-activated phosphors generally exhibit a very short emission decay time .tau..sub.1/e, less than 100 nanoseconds, and accordingly have attracted much attention as favorable to flying-spot scanners and beam-indexing tubes. An emission spectrum extending over the entire range of visible part is required of a phosphor for flying-spot-scanners. For beam-indexing tubes, it is required of a phosphor that the peak wavelength .lambda..sub.max of its emission spectrum is within the range of about 340-400 nm.
Examples of commercial phosphors which meet, or nearly meet, these requirements are a Ca.sub.2 MgSi.sub.2 O.sub.7 :Ce phosphor (designated as P16) whose emission spectrum has a peak .lambda..sub.max approximately at 370 nm, a Y.sub.3 Al.sub.5 O.sub.12 :Ce phosphor (P46) whose .lambda..sub.max is approximately 540 nm, a Y.sub.2 SiO.sub.5 :Ce phosphor (P47) with .lambda..sub.max at approximately at 410 nm and P48 which is a 70/30 mixture of P46 and P47.
Y.sub.2 O.sub.3 -Al.sub.2 O.sub.3 system can present three different phases; Y.sub.3 Al.sub.5 O.sub.12, YAlO.sub.3 and Y.sub.4 Al.sub.2 O.sub.9. Among phosphors given by activation of these yttriumaluminates by cerium, the most greatest interest has been attached to Y.sub.3 Al.sub.5 O.sub.12 :Ce(P46) because Y.sub.3 Al.sub.5 O.sub.12 is the most stable among the three phases and can be obtained readily as a single phase through a simple solid state reaction.
Weber reported in J. of Appl. Phys., 44, 3205-3208 (1973) that the photoemission band of a single crystal of YAlO.sub.3 :Ce prepared by the Czochralski method had a decay time .tau..sub.1/e of 16 nsec and a peak wavelength in the emission spectrum .lambda..sub.max of 370 nm. These values suggest that YAlO.sub.3 :Ce phosphor is quite attractive as a beam-indexing phosphor. Some patent applications, e.g. Japanese Patent Application Disclosure No. 48(1973)-26686, have also disclosed YAlO.sub.3 :Ce phosphor. Unlike Y.sub.3 Al.sub.5 O.sub.12, however, YAlO.sub.3 is a quasi-stable phase and requires some consideration of preparation techniques. Naka et al reported in Kogyo Kagaku Zasshi (Japan), 69, No. 6,1112-1116(1966) that, in the preparation of YAlO.sub.3 from Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3, there occurred coexistence of Y.sub.3 Al.sub.5 O.sub.12 and Y.sub.4 Al.sub.2 O.sub.9 even by the employment of a very severe baking condition such as 1500.degree. C.-25 hr. The above-referred Japanese Pat. Appln. too shows the necessity of employing a baking temperature higher than 1550.degree. C. for obtaining a YAlO.sub.3 :Ce phosphor of a practicable performance. According to the results of our experiments on the production of YAlO.sub.3 :Ce phosphor by solid state reaction, the coexisting Y.sub.3 Al.sub.5 O.sub.12 :Ce phase lessens as the baking temperature is raised but does not disappear completely even through baking for 4 hr at 1600.degree. C.
The coexistence of Y.sub.3 Al.sub.5 O.sub.12 :Ce in YAlO.sub.3 :Ce phosphor as an ultraviolet-emitting phosphor is particularly and significantly disadvantageous from the following reasons. First, Y.sub.3 Al.sub.5 O.sub.12 :Ce emits visible light. Second, the excitation band of Y.sub.3 Al.sub.5 O.sub.12 :Ce somewhat overlaps the emission band of YAlO.sub.3 :Ce with the result that part of the ultraviolet radiation emitted by YAlO.sub.3 :Ce is absorbed in Y.sub.3 Al.sub.5 O.sub.12 :Ce.
Concerning the production of YAlO.sub.3 :Ce phosphor using Y.sub.2 O.sub.3, Al.sub.2 O.sub.3 and CeF.sub.3 as starting materials, our above-referred application Ser. No. 691,784 has disclosed that a phosphor of improved luminous intensity can be obtained through baking at about 1000.degree.-1400.degree. C. by the addition of a controlled amount of barium compound, e.g. BaCO.sub.3, BaO or BaF.sub.2, to the starting materials.
Japanses Patent Application Disclosure No. 49(1974)-113784, relating to Y.sub.2 SiO.sub.5 :Ce phosphor, shows that the addition of BaF.sub.2 to the starting materials for this phosphor causes an increase in the luminous intensity due to an intensification of radiation approximately at 450 nm while there occurs no enhancement of a main peak (approximately at 420 nm) of Y.sub.2 SiO.sub.5 :Ce. This means a change in the emission spectrum of Y.sub.2 SiO.sub.5 :Ce by the use of BaF.sub.2. For YAlO.sub.3 :Ce, we have confirmed that the use of a barium compound causes enhancement of its main peak (370 nm) without causing any change in the emission spectrum.
Japanese Patent Application Disclosure No. 50(1975)-45790 relating to Y.sub.2 Si.sub.2 O.sub.7 :Ce shows an increase in the luminous intensity caused by the use of BaF.sub.2. As is known, Y.sub.2 Si.sub.2 O.sub.7 has four crystallographic modifications; .alpha., .beta., .gamma. and .delta.. This Japanese application describes that the use of BaF.sub.2 is effective for obtaining a single phase of .beta.-Y.sub.2 Si.sub.2 O.sub.7 :Ce which exhibits the most intense emission.
Since YAlO.sub.3 has only one crystallographic structure, the effect of BaF.sub.2 on YAlO.sub.3 :Ce phosphor shown in our application Ser. No. 691,784 differs in nature or principle from that on Y.sub.2 Si.sub.2 O.sub.7 :Ce phosphor.
For Y.sub.3 Al.sub.5 O.sub.12 :Ce, we have recognized through later studies that the addition of a barium compound to commonly used starting materials produces a suppressive effect on the formation of Y.sub.3 Al.sub.5 O.sub.12 :Ce by baking. When no barium compound was added, pure Y.sub.3 Al.sub.5 O.sub.12 :Ce phase was obtained at baking temperatures above about 1300.degree. C. and baking at lower temperatures resulted in the coexistence of YAlO.sub.3 :Ce phase. When a few mole% barium compound was added to the same starting materials, the coexistence of YAlO.sub.3 :Ce phase did not expire even at baking temperatures somewhat above 1300.degree. C., so that it was necessary to raise the baking temperature at least to 1400.degree. C. for obtaining pure or nearly pure Y.sub.3 Al.sub.5 O.sub.12 :Ce phase.